U.S. Pat. to Smith No. 3,476,088 of Nov. 4, 1969, shows a cross-contamination prevention device for animal cages. The isolator of the present invention is an improvement thereover, both in its construction and in its operation. The prior device is expensive to fabricate and does not prevent back flow. In the event of sudden loss of pressure or development of suction at its inlet, the air in the storage chamber of the prior device could be sucked back into the standpipe and draw with it liquid from the storage reservoir to contaminate the liquid supply source and other animal cages.